Gas-liquid mixing is an important and common unit operation in chemical and allied industries. A common manner of introducing gas to liquid is by injecting compressed gas at a certain depth below the liquid surface through orifices or porous headers. Gas is dissolved by the continuous contacting of the upward moving gas bubbles through the liquid. It is well known that the efficiency in terms of mass of transferred gas per unit power is very low and maintenance cost of such system is high due to the frequent plugging of the distributing headers. This is especially the case when the gas-liquid reaction takes place in the presence of solid suspensions, for example, the aerobic reaction of biological treatment of wastewater. Another disadvantage of this gas-liquid contact system is that it requires very complicated mechanical components which includes gas blowers, gas filtration units, special building for housing the blower and extensive piping and manifold for distributing the gas flow.
Another known means for affecting gas-liquid mixing involves injecting compressed gas through spargers and employing a submerged turbine for breaking the injected gas into small bubbles. In the submerged turbine system gas contacts with liquid in an intensive turbulence zone, and, as a consequence, the efficiency of gas mass transfer is relatively higher. Since the gas is injected through larger sparger openning, the plugging problem is mitigated and therefore eliminating the need for filtration unit. However, the mechanical system is extremely complicated and expensive, since it includes a rotational submerged turbine unit in addition to the gas compression unit.
Jet aeration is another method for transferring gas in liquid which involves recirculating of a portion of the reactor liquid and mixing it with compressed gas in a jet assembly before injecting into the liquid body. The jet aeration system has rather high gas absorption rate and high mass transfer efficiency. However it requires a liquid pumping system in addition to a gas compression system.
It is worthwhile to note that the efficiencies of various gas-liquid transfer systems depend on the depth of the gas injection. In general, mass transfer efficiency and gas absorption rate increases with the injection depth. This is mainly due to the longer retention time of the injected gas bubbles in the liquid and the better utilization of the power employed for gas compression. However there are also limitations and tradeoffs. For example, a commercial centrifugal gas blower can deliver only a given maximum gas pressure which limits the depth of gas penetration. For deeper gas injection extra cost must be spent on multi-stage or more expensive type special gas compressors.